Resource scheduling in wireless communication systems using beam forming

ABSTRACT

A basic idea of the invention is to provide multi-user resource scheduling and distribution based on balancing the power resources used for the different narrow beams in order to smooth the interference levels over the whole cell, area and to reduce interference fluctuations. The resource scheduling principle according to the invention is especially useful when the available resources are not fully utilized. The idea is to select, for each antenna beam of at least a subset of the antenna beams, at least two mobile users for service using the respective antenna beam during a transmission time interval, and to distribute power resources to the antenna beams for use during the transmission time interval based on the guideline of balancing the power resources among different antenna.

This application claims the benefit of U.S. Provisional Application No.60/818,980, filed Jul. 7, 2006, the disclosure of which is fullyincorporated herein by reference.

TECHNICAL FIELD

The present invention generally relates to wireless or cellularcommunication and more particularly to advanced resource scheduling.

BACKGROUND

The use of adaptive or smart antennas is considered as one of the keyfeatures for increasing coverage and capacity of a wireless or cellularsystem. When beamforming is applied in the base station several narrowbeams, compared to the sector/cell beam, may be created to maintaincoverage in the cell. FIG. 1A illustrates a sector cell antenna beam.Although a sector antenna is useful to communicate broadcast and/orcontrol information to all mobiles in the sector cell, an adaptiveantenna may be used to transmit and receive in narrow beams coveringjust a part of the sector cell. FIG. 1B shows an example of a narrowantenna beam. FIG. 2 illustrates an example of a cellular network with abase station transmitting a sector beam, a base station transmitting oneof the possible beams in a multi-beam system, and a base stationtransmitting a steerable beam. Some benefits of adaptive antennas areshown in FIG. 3, where a narrow beam of the adaptive antenna may bedirected to an intended mobile and therefore spreads less interferencein the download direction. The narrow beam also suppresses spatialinterference from adjacent cell interferers in the uplink direction.Both factors increase the signal-to-interference gain, and thereforeincrease the overall system performance.

HSDPA (High Speed Data Packet Access) is another important feature thatenables improvements in capacity and end-user perception by means ofefficient sharing of common resources in the cell among many users,rapid adaptation of the transmission parameters to the instantaneousradio channel conditions, increased peak bit rates and reduced delays.

Fast scheduling is the mechanism selecting which users to transmit to ina given transmission time interval (TTI). The scheduler is a key elementin the design of a HSDPA system as it controls the allocation of theshared resources among the users and to a great extent determines theoverall behavior of the system. In fact, the scheduler decides whichusers to serve and, in close cooperation with the link adaptationmechanism, which modulation, power and how many codes should be used foreach user. This produces the actual end-users bit rate and systemcapacity.

In order to improve the system capacity in terms of total cellthroughput, the scheduling algorithm normally bases its decisionprimarily on the channel conditions experienced by the user equipment(UE); however, it can be designed in a flexible way so as to considerother aspects such as the fairness from a time resource perspective orthe average bit rate.

In a WCDMA HSDPA system, a channel quality indication (CQI) is reportedby the UE and used for scheduling and link adaptation. Since the CQI isstrictly connected to the quality of a common pilot signal, it isstrongly affected by the interference levels.

The fast scheduler, normally located in the radio base station (RBS),targets which users to serve in each TTI and distributes the availablechannelization codes and power resources among the selected users. Incase code multiplexing is applied, more than one user can be served inthe same TTI by using distinct parts of the set of channelization codesallocated for the HSDPA related channels. Several scheduling algorithmsthat can be used to enforce specific strategies in terms of trade-offbetween system capacity and user fairness are available in theliterature. The most common algorithms are described below.

The round robin (RR) algorithm allocates radio resources to the users ona sequential basis and it does not base its decision on theinstantaneous radio channel conditions experienced by the connection.The system performance is not maximized even though a certain degree offairness is obtained in terms of access to the radio resources.

The proportional fairness (PF) scheduler better exploits the channelconditions and ensures that all users receive a guaranteed minimumthroughput, providing fairness among users together with systemperformance improvement. The scheduler transmits information to someusers based on CQI information, delay and other measurements.

The maximum C/I algorithm bases the user selection solely on the CQIinformation reported by the UE.

The CQI report in WCDMA multi-beam antenna system is based on thequality of the Secondary Common Pilot Channel (S-CPICH).

However, existing solutions do not enable the system to benefit from thecapacity/coverage gains promised by narrow beam/adaptive antennatechniques.

SUMMARY

It is a general object of the present invention to improve schedulingfor cellular communication using multi-beam antenna systems.

It is an object to improve the possibility to secure and achieve thesystem capacity/coverage gains promised by narrow beam/adaptive antennatechniques, and efficiently explore the available radio resources.

It has been recognized that a problem of a scheduling algorithm appliedin a multi-beam antenna system is how to manage the interferencevariations. For example, existing solutions do not enable the system tobenefit from the spatial interference filtering provided by the narrowbeams and are not designed to avoid large interference variations causedby sudden changes in the transmission power of different narrow beams.This may lead to incorrect detection of the received data at the userequipment side, a lot of retransmissions and, in the end, inefficientexploration of the available radio resources.

The present invention addresses this and similar/associated problems,and proposes an advanced Resource Scheduling Mechanism for Multi-beamAntenna Systems based on beam power balancing. The invention isgenerally applicable to resource scheduling, but particularly useful inHSDPA systems with narrow beam capabilities.

A basic idea of the invention is to provide multi-user resourcescheduling and distribution based on balancing the power resources usedfor the different narrow beams in order to smooth the interferencelevels over the whole cell area and to reduce interference fluctuations.The resource scheduling principle according to the invention isespecially useful when the available resources are not fully utilized.The idea is to select, for each antenna beam of at least a subset of theantenna beams, at least two mobile users for service using therespective antenna beam during a transmission time interval, and todistribute power resources to the antenna beams for use during thetransmission time interval based on balancing the power resources amongdifferent antenna.

It should be understood that the beam balancing condition is intendedfor use as a guideline when distributing power resources to thedifferent antenna beams.

In general, a benefit of the beam balancing principle in multi-userresource scheduling is that for example CQI measurement reports from theUEs in the coverage area of the cell will be more reliable, and willprovide a good basis for fast scheduling and fast link adaptation. Inaddition, the spatial filtering provided by the narrow beams reduces theabsolute levels of interference generated within the cell, leading tosubstantially improved throughput rates as well as individual peak ratesboth in the specific cell as well for the neighboring cells.

Other advantages offered by the invention will be appreciated whenreading the below description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a sector cell antenna beam.

FIG. 1B shows an example of a narrow antenna beam.

FIG. 2 illustrates an example of a cellular network with a base stationtransmitting a sector beam, a base station transmitting one of thepossible beams in a multi-beam system, and a base station transmitting asteerable beam.

FIG. 3 illustrates how a narrow beam of an adaptive antenna may bedirected to an intended mobile and therefore spreads less interferencein the download or downlink direction.

FIG. 4 is a schematic flow diagram illustrating a preferred exemplaryembodiment of the invention.

FIG. 5 is a schematic diagram illustrating an example of a multipleantenna beam transmission in a transmission time interval.

FIG. 6 is a schematic diagram illustrating a radio base stationresponsible for a cell split into four narrow beams and seven users tobe served in a certain transmission time interval.

FIG. 7 illustrates relevant rudimentary portions of a radio base station(RBS) according to an exemplary embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

For a better understanding of the invention, it may be useful to beginwith a brief overview and analysis of the prior art techniques.

As mentioned, the inventors have recognized that a primary problem of ascheduling algorithm for a multi-beam antenna system is how to managethe interference variations. The existing conventional solutions do notenable the system to benefit from the spatial interference filteringprovided by the narrow beams and are typically not designed to avoidlarge interference variations caused by sudden changes in thetransmission power of different narrow beams. In case that happens, theCQI used for the scheduling and link adaptation decisions may notreflect the actual interference level when the user is actuallyscheduled, leading to incorrect detection of the received data at theuser equipment side, a lot of retransmissions and, in the end,inefficient handling and poor utilization of the available radioresources.

The unbalanced transmission in the different beams also increases themomentary interference peaks towards neighboring cells and affects theirperformance.

More specifically, the interference levels depend on several factorssuch as: the position within the cell, the fast fading, theinstantaneous transmission activity of neighbor cells and, in case of atime-dispersive channel, the transmission activity of the own cell. Theinstantaneous own cell interference can be significant in case ofapplication of beamforming, as a large amount of the RBS power can besteered towards a specific narrow beam. In that case, the CQI reportedby the UEs in that beam will degrade compared with a situation where thepower is more balanced among different beams. Another important issue isthat the scheduling and link adaptation must rely on “reliable” CQI fromthe UEs. This might not occur if there are sudden interference levelvariations due to the fact that all the RBS power is transmitted in onebeam during a certain TTI and in another beam in the next TTI.Unreliable CQI reports imply incorrect scheduling and link adaptationdecisions which in turn result in a lower capacity, andunder-utilization of available resources if a channel is believed to beworse than is really the case.

US patent application US 2006/0067269 published on Mar. 30, 2006 relatesto a method of scheduling users in wireless communication networks,where the selection of additional users of a group within a sector of acell, to be simultaneously scheduled with already scheduled user(s) ofthe same group, may be based on spatial information of the users in thegroup. The spatial information for each user within a sector of a cellincludes an incidence angle of arrival of a transmission signal to theuser with respect to a 0° sector border.

The US 2006/0067269 patent application mainly concerns the so-calledcode limitation problem.

US patent application US 2005/0064872 published on Mar. 24, 2005concerns reducing shared downlink radio channel interference bytransmitting to multiple mobiles using multiple antenna beams. Multiplemobiles are selected to receive transmissions over the shared radiochannel during a predetermined transmission time interval. The schedulerselects one mobile per each antenna beam. The scheduler also monitorsthe available channel resources, and employs some type of resourceallocation method for allocating resources for each antenna beamtransmission for each transmission time interval. The radio resourcesare divided evenly between the selected mobile radios, or alternativelydivided in proportion to each mobile's reported channel quality.

Although US patent application US 2005/0064872 provides a quitesatisfactory solution there is still room for improvement as will beexplained below.

The present invention addresses this and similar/associated problems,and proposes an advanced Resource Scheduling Mechanism for Multi-beamAntenna Systems based on beam power balancing. This mechanism ispreferably implemented as a method and a corresponding arrangement orsystem. A basic concept of the present invention is to balance the powerresource used for the different narrow beams (preferably in every TTI)in order to smooth the interference levels over the whole area coveredby the cell and to reduce interference fluctuations. Reference can bemade to the schematic flow diagram of FIG. 4.

The proposed multi-user resource scheduling mechanism is based on thegeneral principle of selecting multiple mobile users to be scheduled forservice using multiple narrow antenna beams during a predeterminedtransmission time interval, in similarity to co-pending US patentapplication US 2005/0064872 mentioned above. However, here the idea isto select, for each antenna beam of at least a subset of the antennabeams of the radio base station, multiple (i.e. at least two) mobileusers for service using the respective antenna beam during atransmission time interval (S1), rather than a single user per beam. Ifthere are available resources, the idea according to the invention isnormally to allocate multiple mobile users per beam (at least for asubset of the beams). Furthermore, in accordance with the invention, thepower resources are distributed to the antenna beams for use during theconsidered transmission time interval based on balancing the powerresources among different antenna beams (S2), rather than distributingthe power evenly among the users.

Understanding that the beam balancing principle is normally used as aguideline, the mobile users served by a cell may for example bescheduled so that the number of users or the corresponding totaltransmitted power is “evenly” or “homogeneously” distributed among/overthe different narrow beams of the cell to provide a more homogeneous or“spatially white” interference situation.

With multiple scheduled users per beam (for at least a number of thebeams) per TTI, it is often better to distribute the power resourcesbased on balancing the power resources among the beams, rather thandistributing the power evenly among the users, as can be understood fromthe following illustrative example.

For a better understanding of the invention it may be useful to consideran illustrative example with reference to the schematic diagram of FIG.5. Consider five different narrow beams B1, B2, B3, B4 and B5, witheight users u1-u8 scheduled for service during a transmission timeinterval. In this particular example, two users (u1, u2) are scheduledin B1, one user (u3) in B2, two users (u4, u5) in B3, one user (U6) inB4 and two users (u7, u8) in B5.

If the power resources would be distributed evenly among the eightusers, each user would be allocated 12.5% of the total available power.In effect, this would mean that B1 (having two users) is assigned 25%,B2 (with only one user) is assigned 12.5%, B3 (two users) is assigned25%, B4 (one user) is assigned 12.5% and B5 (two users) is assigned 25%of the power resources. As can be seen there is a quite significantdifference between the different beams. B1, B3 and B5 have twice thepower of B2 and B4.

However, by using the principle of beam balancing the guideline will beto assign 20% of the power resources to each beam. In practice, this mayfor example turn into something like 21.4% to B1, 18.7% to B2, 20.2% toB3 and 21.8% to B4, and 17.9% to B5, which results in a much morebalanced and smoothed interference situation in the entire cell.

For comparison, it should be understood that with the scheme proposedaccording to US patent application US 2005/0064872 only five of theeight mobile users are selected to receive transmissions during atransmission time interval; one mobile per beam.

Normally, the invention aims at balancing the power resources within anygiven transmission time interval. Alternatively, however, the inventionaims at balancing the power resources averaged over a period of severaltransmission time intervals.

The invention can also be adapted to handle situations when there are noactive users in one or more beams, while still adhering to the overallprinciple of beam balancing. For example, if there are no active mobileusers in a given beam, more power resources can be allocated to antennabeams that are direct neighbors of the given antenna beam than to moredistant antenna beams to balance or smooth the power resourcedistribution over the whole intended coverage area of the cell. Due tothe normal beam overlap, the increased power in the neighboring “active”beams will “spill over” to the “inactive” beam and provide the desiredsmoothing.

Consider a small modification of the above example, assuming that thereare no active users at all in B2, meaning that user u3 is not activeduring a given period of time. Then it may in fact be desirable toincrease, at least for the given period of time when user u3 isinactive, the power resources in B1 and B3, for example to 30% of thepower resources, resulting in an exemplary beam balancing guideline of30% to B1, 30% to B3 and 20% to B4, and 20% to B5, assuming that B1 andB3 would “spill over” power to the “inactive” beam B2. Overall,considering the intended coverage area of the entire cell, this mayresult in a more smoothed and balanced interference situation, ratherthan giving 25% to each of the “active” beams B1, B3, B4, and B5.

In general, a benefit of the beam balancing principle in multi-userresource scheduling is that the CQI measurement reports from the UEs inthe coverage area of the cell will be more reliable, and will provide agood basis for fast scheduling and fast link adaptation. In addition,the spatial filtering provided by the narrow beams reduces the absolutelevels of interference generated within the cell, so the UEs will reporthigher CQIs (on average) and will achieve higher bit rates. Finally, byperforming beam balancing within one or more cells, the interferencepatterns towards neighboring cells may also be smoothed, potentiallyleading to better performance for those cells as well.

The invention is generally applicable to resource scheduling, butparticularly useful in HSDPA systems with narrow beam capabilities.

In a preferred exemplary embodiment of the invention, the selection ofusers in the overall scheduling mechanism works based on a ranking listof mobile users in a cell, while at the same time keeping track ofspatial information represented by the narrow antenna beam in which eachmobile user is located. Preferably, the selection procedure starts byselecting a best ranked mobile user from the ranking list for service bya given narrow antenna beam. Then the ranking list is traversed toselect the following users not only by looking at the position in theranking list but also by considering the spatial information so that anext user down in the list is located in another narrow antenna beamdifferent from that of previously selected user(s) higher up in theranking list. Once the end of the list is encountered the selectionprocedure starts over from the top of the list to select further mobileusers, preferably until all considered users have been selected and/orthe available power resources have been allocated.

The proposed advanced solution can for example be applied on top of oneof the existing basic solutions (e.g. RR, PF, max C/I). It is forexample assumed that the basic algorithms select the users to bescheduled in each TTI by picking the best ranked users in a per-cellpriority list (obtained according to a certain scheduling function).However, the advanced solution of the invention also considers the“spatial” information, i.e. which beam the UE is located in, and aims at“balancing” (at least approximately) the power resources used in eachbeam in order to even out the interference levels in the cell.

The proposed solution can be illustrated with another example,considering the simple scenario outlined in FIG. 6; a single cell splitin four narrow beams and seven users to be served in a certain TTI.Reference can also be made to FIG. 7, which illustrates the relevantrudimentary portions of an arrangement in a radio base station (RBS)according to an exemplary embodiment of the invention. In this example,the radio base station 100 basically comprises a scheduler 10 andassociated data bases 20, and standard multi-beam transceiverfunctionality 30 with associated data buffers (not shown).Alternatively, the proposed scheduler may be implemented in anothernetwork node such as the radio network controller (RNC) or distributedbetween several nodes such as the RNC and RBS nodes.

The advanced scheduling algorithm creates a ranking list 22 (accordingto a certain function, e.g. based on CQI or equivalent qualityindication) and also keeps track (in a separate record 24 or integratedwith the ranking list 22) of the spatial information represented by thenarrow beam in which the user equipment (UE) is located. Thisinformation is made available to the scheduler 10, e.g. by keeping trackon which S-CPICH (or equivalent the cell-portion in 3GPP NodeBnomenclature) that the specific UE is associated to at channelassignment and activation by the radio network controller (RNC).

Table 1 below illustrates an example of a ranking list table includingspatial information for the simple scenario exemplified in FIG. 6.

User Beam/Cell portion/ Equipment S-CPICH Index UE5 1 UE3 1 UE1 2 UE4 4UE6 4 UE2 3 UE7 2

The scheduling procedure executing in the scheduler selects the nextusers not only by looking at the position in the ranking list but alsoby considering the spatial information. A basic exemplary balancingcriterion is to equalize as much as possible the number of users servedin each beam.

The balancing criteria can be further enhanced by considering theexpected total transmitted power in each beam rather than the number ofusers served in each beam. This enhancement ensures a direct controlover the interference generated in each beam even if it requires moreinformation, such as measurements of the total power per beam.

Referring to FIG. 6 and Table I, the scheduler may first select the UEat the top of the list (UE5) and the RBS then assigns the power andchannelization code resources to be used. However, instead of selectingUE3, the next users selected by the scheduler after UE5 could forexample be: UE1, UE4, and UE2.

In other words, an exemplary procedure could start by selecting the best(e.g. with the best CQI) ranked user of all considered users. This useris located in a specific cell portion #1 (served by a correspondingnarrow beam) as identified by the spatial information (e.g. S-CPICH).Instead of simply selecting the second best ranked user, the schedulerfinds the best user which is located in another cell portion. Thescheduler goes down the ranking list to find the next cell portion #2and selects the user equipment UE1 at that position in the list. Theprocedure continues by traversing the list towards lower rankings tofind the next cell portion #4 and selecting the corresponding userequipment UE4. Subsequently, the next cell portion is #3 and thecorresponding user equipment is UE2. The scheduler can now start overand then finds UE3 in cell portion #1, UE6 in cell portion #4 and UE7 incell portion #2.

The basic “rule” can be broken in case there are no users and/or radioresources left in one particular beam but still there are radioresources and users to be served in other beams. In that case theadvanced algorithm may continue to balance the resources among theremaining beams. In the example of FIG. 6, there are no users left inthe third beam; however, as explained, the algorithm continues tobalance the resources among the other beams and selects UE3, UE6, andUE7.

An advantage with the proposed solution is that the interference in thesystem will be more predictable and hence the validity of the reportedCQI will increase. When using the scheduling principle disclosed here,the interference seen from other cells will typically appear “whiter” inthe spatial domain. This will then be reflected in the CQI measurementwhere the fluctuations (due to interference) will be much smaller. Thiswill reduce the number of re-transmissions needed since the validity ofa received CQI will be longer. This will in a multi-user schedulingscenario, as a consequence, result (with large probability) in asubstantially improved throughput rate as well as individual peak ratesboth in the specific cell as well for the neighboring cells.

In addition, one or more of the users may have different services (withdifferent priority levels and/or data rate requirements) such as (IP)voice service, e-mail service and/or some form of more advanced mediaservice that are scheduled, for each considered user, according topriority and/or data rate requirements, while still adhering to theoverall principle of beam balancing. In other words, the variousservices are preferably scheduled over time based on the overallprinciple of balancing the power resources among different antenna beamswhile efficiently utilizing the available resources. Depending on theavailable resources, the number and/or types of services that arescheduled for the user(s) based on the beam balancing principle may thusvary over time. One or more users may have to temporarily pause lowpriority services in order to allow activation of more importantservices or more advanced media services for the same or other users.This also means that several mobile users with low data rate servicesmay be scheduled to a particular beam, whereas only a single user with amore advanced service may be scheduled to another beam.

For the interested reader, basic information on HSDPA can be found inthe relevant 3GPP specifications, and on pages 69-92 in the Ph.D. Thesis“Packet Scheduling and Quality of Service in HSDPA” by P. José and A.Gutiérrez, October 2003.

The embodiments described above are merely given as examples, and itshould be understood that the present invention is not limited thereto.Further modifications, changes and improvements which retain the basicunderlying principles disclosed herein are within the scope of theinvention.

The invention claimed is:
 1. A method for multi-user resource scheduling in a radio communication system having a radio base station with narrow beam capabilities, wherein said method is based on selecting multiple mobile users to be scheduled for service using multiple narrow antenna beams during a predetermined transmission time interval, characterized by: selecting, for each antenna beam of at least a subset of said multiple antenna beams, at least two mobile users for service using the respective antenna beam during said transmission time interval; and distributing power resources to the antenna beams for use during said transmission time interval based on balancing the power resources among different antenna beams, wherein said narrow antenna beams are partially overlapping, and said step of distributing power resources to the antenna beams comprises the step of allocating, when there are no active mobile users in a given beam, more power resources to antenna beams that are direct neighbors of the given antenna beam than to more distant antenna beams to balance the power resource distribution over the whole area covered by the cell.
 2. The method of claim 1, wherein the power resources are distributed among the different antenna beams in order to smooth the interference levels over the whole area covered by the cell and to reduce interference fluctuations.
 3. The method of claim 1, wherein said step of selecting comprises the steps of providing a ranking list of mobile users in a cell and at the same time keeping track of spatial information represented by the narrow antenna beam in which each mobile user is located.
 4. The method of claim 1, wherein said selecting step comprises the steps of selecting a best ranked mobile user from the ranking list for service by a given narrow antenna beam, and then traversing the ranking list to select the following users not only by looking at the position in the ranking list but also by considering the spatial information so that a next user down in the list is located in another narrow antenna beam different from that of previously selected user(s) higher up in the ranking list, and once the end of the list is encountered the selection procedure starts over at least one time from the top of the list to select further mobile users.
 5. The method of claim 1, wherein said steps of selecting and distributing are performed for each transmission time interval.
 6. The method of claim 1, wherein at least one user is associated with a number of different services, and these services are scheduled based on balancing the power resources among different antenna beams such that the number and/or types of services that are scheduled for said at least one user vary over time in dependence on the available resources.
 7. An arrangement for use in a cellular radio communication system, said arrangement configured for generating multiple antenna beams associated with a communication cell, each beam covering only a portion of the cell, the arrangement further configured for selecting multiple mobile users to be scheduled for service using multiple antenna beams during a predetermined transmission time interval, the arrangement comprising: a processor configured for: selecting, for each antenna beam of at least a subset of said multiple antenna beams, at least two mobile users for service using the respective antenna beam during said transmission time interval; and distributing power resources to the antenna beams for use during said transmission time interval based on balancing the power resources among different antenna beams, wherein said narrow antenna beams are partially overlapping, and said processor is further configured for allocating, when there are no active mobile users in a given beam, more power resources to antenna beams that are direct neighbors of the given antenna beam than to more distant antenna beams to balance the power resource distribution over the whole area covered by the cell.
 8. The arrangement of claim 7, wherein said processor is further configured for providing a ranking list of mobile users in a cell and for keeping track of spatial information represented by the narrow antenna beam in which each mobile user is located.
 9. The arrangement of claim 7, wherein said processor is further configured for selecting a best ranked mobile user from the ranking list for service by a given narrow antenna beam, and traversing the ranking list to select the following users not only by looking at the position in the ranking list but also by considering the spatial information so that a next user down in the list is located in another narrow antenna beam different from that of previously selected user(s) higher up in the ranking list, and once the end of the list is encountered starting the selection procedure over at least one time from the top of the list to select further mobile users.
 10. The arrangement of claim 7, wherein said arrangement is implemented in at least one network node of said radio communication system.
 11. The arrangement of claim 10, wherein said at least one network node includes a radio base station. 